Mass spectrometry (MS) is a powerful analytical technique due to its sensitivity, versatility, and ability to provide chemical and structural information of molecules; because of this, it is often the detection method of choice for a wide range of applications. Electrospray ionization (ESI) has significantly expanded the range of mass spectrometric analysis to include biomolecules and other liquid-borne analytes. ESI provides a facile method for coupling liquid phase separations, such as liquid chromatography (LC) or capillary electrophoresis (CE) with MS detection. As a result, LC-MS has become a widely used analytical tool in fields such as proteomics, environmental monitoring, drug discovery and development, and clinical diagnostics. However, conventional LC-MS systems are usually confined to dedicated laboratories because they are large, expensive, complex, and require significant amounts of power. Conventional mass spectrometers are unsuitable for these situations because of their large size, weight, and power consumption (SWaP). See, e.g., Whitten et al., Rapid Commun. Mass Spectrom. 2004, 18, 1749-52. Miniaturization of LC-MS systems is limited by the need for a rugged system of pumps, valves, and tubing, while mass spectrometers are limited by low pressure operation, which have conventionally required bulky, fragile, and expensive turbomolecular pumps.
One of the difficulties associated with coupling ESI sources with MS systems is that ions must be transported into vacuum for mass analysis. See, e.g., Page J. S., et al. “Ionization and Transmission Efficiency in an Electrospray ionization—mass Spectrometry Interface.” J. Am. Soc. Mass. Spec., 2007, 18(9), 1582-1590. The transmitted ion current from an ESI source through a capillary inlet system can be reduced by up to three orders of magnitude. These losses occur mostly in transfer regions from a higher pressure to lower pressure (i.e. on either side of a capillary inlet) and two or more of these regions are typically used in traditional ESI-MS. See, e.g., S. A. Shaffer, K. Tang, G. A. Anderson, D. C. Prior, H. R. Udseth, R. D. Smith. Rapid Communications in Mass Spectrometry, 1997, 11, 1813-1817. This presents a significant challenge for coupling ESI with HPMS.